Processes for the conversion of synthesis gas, a mixture comprising carbon monoxide and hydrogen, into hydrocarbons have been known for some time, of which the Fischer-Tropsch process is a well known example. A description of the process may be found, for example, in Kirk Othmer: Encyclopedia of Chemical Technology, 3rd Edition, Vol. 11, pages 473-478 (John Wiley & Sons, New York, N.Y. 1980). In the Fischer-Tropsch process the synthesis gas is passed over a metal oxide catalyst at elevated temperatures and pressures, typically in the range of 250.degree. C. to 500.degree. C. (about 480.degree. F. to 930.degree. F.) and 1000 to 30,000 kPa (about 130 to 4340 psig); the catalyst is usually derived from a metal such as iron, cobalt, nickel or thorium. The product of the Fischer-Tropsch process is typically a mixture of hydrocarbons whose composition depends upon the nature of the catalyst employed and the selected process conditions.
Other catalysts for converting synthesis gas into hydrocarbons have also been proposed in the past. For example, U.S. Pat. No. 4,207,248 discloses a catalyst comprising a mixture of a Fischer-Tropsch component and a zeolite such as ZSM-5. U.S. Pat. Nos. 4,207,250, 4,255,349 and 4,298,695 describe the use of catalysts comprising iron, an acidic zeolite such as ZSM-5 and a matrix, with all three components combined in individual particles of the catalyst. U.S. Pat. No. 4,207,248 discloses a similar catalyst which employs cobalt instead of iron.
Proposals have been made for increasing the proportionate yield of certain kinds of hydrocarbons in the product. For example, U.S. Pat. No. 4,279,830 refers to the possibility of obtaining an improved yield of aromatics by using a iron catalyst promoted with potassium in combination with zeolite ZSM-5. The cobalt-containing catalyst of U.S. Pat. No. 4,207,248 mentioned above, is said to produce an improved yield of olefins, with a proportionately reduced yield of lower hydrocarbons such as methane and ethane. Similarly, the processes of U.S. Pat. Nos. 4,207,250, 4,255,349 and 4,208,695 are said to produce good yields of their desired higher molecular weight products while minimizing the yields of methane and other low molecular weight hydrocarbons. On the other hand, the process described in U.S. Pat. No. 4,177,202 employs a transition metal catalyst which is selective to a certain degree for the production of light gases rich in methane and ethane. The highest reported yield of ethane is 44 percent of the total hydrocarbon product and although this figure is encouraging it falls short of the amount which would be desirable in such a process.